ROTATING-RING-DISK ELECTRODE STUDIES OF POLYPYRROLE-GLUCOSE OXIDASE BIOSENSORS

A rotating ring-disk electrode, (RRDE) of which the platinum disk was coated with a polypyrrole-glucose oxidase (PPy-GOD) film by electrochemical polymerization and the ring was left bare, has been used to characterize the PPy-GOD electrode. Polypyrrole-GOD films of various thicknesses grown in the presence of 100 and 500 U/ml of GOD in the deposition solution were used. The amperometric current generated at 0.7 V versus saturated calomel electrode (SCE) by the electrochemical oxidation of hydrogen peroxide, generated by the enzymatic reaction, has been evaluated at both the coated disk and uncoated ring at various rotation speeds. The collection efficiency of the RRDE was 22% and was used to evaluate the total current of the PPy-GOD electrode. The total current of a polypyrrole electrode was defined as the sum of the disk current and the total ring current taken as the observed ring current divided by the collection efficiency of the RRDE. From this study, it clearly appears that the biotransformation of glucose takes place mainly at the polypyrrole film/solution interface independently of the rotation speed and that the catalysis is not limited by oxygen. The Eadie-Hofstee type plots obtained for disk current with 10-225 mM of glucose show a strong positive deviation for glucose concentrations higher than 100 mM. On the other hand, linear plots are obtained when the total current of the enzyme electrode is considered. No definitive explanation can be given for these behaviors, except that at a higher glucose concentration, the flux of glucose is high enough to permit the diffusion of glucose deeper in the film, where it can be oxidized by the enzyme. Thus, the catalytic reaction is taking place in a thicker layer of the film.